With a sharded cluster, what is best: CNAME round-robin or seed list


Hi there,

I use Meteor with a MongoDB sharded cluster (with no oplog tailing, and slight modifications to Meteor to disable unique indices and allowing multiple servers in seed list with no master).

What is best:

  • use CNAME round-robin to ‘load-balance’ (and failover?) between mongos, or
  • use multiple mongos servers in the connection string



Congrats on getting the cluster sharded. Last big data project I worked on, we only got the two replica sets up, but didn’t get the control servers up and going. But we were leaning towards CNAME round-robin, from a devops perspective. Easier to swap out servers during maintenance if they have that CNAME abstraction in there.


Hi Abigail and thanks for the reply. Do you confirm that multiple CNAME is well handled by mongo’s driver in case of failover?


It’s been a year or so since I was last working in this area, but as I recall, our initial tests were that it worked. I wouldn’t say that we did extensive or exhaustive testing; so there may edge cases that I don’t know about. But tentatively, yes; as I recall, we installed CNAME aliases, and everything worked just fine. It was not having a DNS abstraction layer that was causing problems with cluster management, and getting the CNAMES in place was a bit of a logistical headache with our hosting provider, and something we considered a bit of a milestone. Shortly after we got them in place, and did a few up/down tests, confirmed everything was working, I left the project. But CNAMES were what we had identified as the preferred approach.


Great, thanks for your feedback!